Carbon dioxide laser light has been widely used for the processing of an object to be processed because carbon dioxide laser light can easily and stably provide a laser output required for the processing (e.g., cutting) of an object to be processed such as a metal member. Carbon dioxide laser light generated from an oscillator of a carbon dioxide laser processing apparatus is generally linearly polarized. However, when irradiating an object to be processed with this linearly polarized carbon dioxide laser light, a problem in processing quality may occur because a processed width may vary depending on polarization direction of the carbon dioxide laser light.
Then, in a carbon dioxide laser processing apparatus, in order to eliminate the influence of the polarization direction of the carbon dioxide laser light on the processing quality, a circular polarization mirror is provided that is a reflection-type optical component, thereby converting the polarization state of the carbon dioxide laser light from a linear polarization to a circular polarization (see Patent Literature 1, for example).